1. Field of the Invention
The invention relates to a display device for use in a computer monitor, a television set or the like, and more particularly, to a display device including a display panel which has three kinds of terminals, i.e., an anode, cathodes and gates, the cathodes and the gates being connected in matrix form.
2. Description of Related Art
In recent years, flat-panel display devices using electron emission elements have been attracting more and more attention.
There are a hot-cathode type of electron emission element and a cold-cathode type of electron emission element. The display panels for flat-panel display devices mainly employ electron emission elements of the cold-cathode type, and a field emission type (hereinafter referred to as the FE type), a metal/insulator/metal type (hereinafter referred to as the MIM type), a surface conduction type (hereinafter referred to as the SC type) and the like are known.
A famous example of the FE type is disclosed in C. A. Spindt, “Physical properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5248 (1976). A known example of the MIM type is disclosed in C. A. Mead, “Operation of Tunnel-Emission Devices”, J. Appl. Phys., 32, 646 (1961). A known example of the SC type is disclosed in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965)
To realize a display panel by using these electron emission elements as its electron sources, there are provided a substrate on which cathodes and gates are formed to be connected in XY matrix form, and an anode having a phosphor layer arranged in opposition to the substrate. The display panel is constructed to irradiate electrons emitted from the electron emitters of the cathodes onto the phosphor layer on the anode and cause the phosphor layer to emit light.
As such electron emission elements, fibrous electron emitters or carbon-based materials which are small in work function for electron emission and are low in threshold voltage are attracting attention, and examples using these electron emission elements are disclosed in Patent Documents 1 to 3.
Any of these examples employs fullerene, diamond, diamond-like carbon (DLC), carbon nanotubes (CNT), fibrous carbon and the like as electron emitters.
In the case of an electron emitter which is low in threshold voltage and uses three kinds of terminals, electrons are emitted from the electron emitter provided on the cathode by field electron emission, merely by applying a normal high voltage (anode voltage) between the anode and the cathode without applying a voltage between the cathode and the gate. Accordingly, it is possible to realize a construction which, during emission, performs electron emission without applying a voltage between the cathode and the gate and, during non-emission, restrains electron emission by applying a cut-off voltage (stop voltage) between the cathode and the gate. This operation will be hereinafter referred to as the normally-on type.
A single electron emission element of the normally-on type employing a carbon fiber electron emitter will be described below.
FIGS. 15A and 15B are diagrammatic views showing different potential distributions of the single electron emission element, and FIG. 15A shows a potential distribution appearing during a driven state in which electrons are being emitted, while FIG. 15B shows a potential distribution appearing during a cut-off state in which electron emission is stopped.
The state shown in FIG. 15A is the driven state in which an electric field larger than a threshold electric field with which electron emission is started is generated for an electron emitter 5 on a cathode 2 by only the voltage between a cathode 2 and an anode 6, thereby causing electron emission. This state is called a normally-on state.
For example, if the threshold electric field of the electron emitter 5 is 3 V/μm, in the case where the anode 6 is provided at a position separated from the cathode 2 by a distance of 2 mm, electron emission is started by applying a voltage of 0 V to the cathode 2 and an anode voltage of 6 kV between the cathode 2 and the anode 6.
Incidentally, a far higher anode voltage may also be applied to realize a suitable normally-on state, and the anode voltage may be determined by an electric field strength capable of providing the required current density, according to the voltage-current characteristics of the electron emission element.
For example, if the required current density can be obtained with an electric field strength of 5 V/μm, an anode voltage of 10 kV may be applied in the case where the anode 6 is provided at a position separated from the cathode 2 by a distance of 2 mm.
FIG. 15A shows the state of equipotential surfaces. In FIG. 15A, equipotential surfaces are nearly uniformly present between the anode 6 and the electron emitter 5, and an electric field strength near the electron emitter 5 is about 5 V/μm, whereby electron emission occurs.
In addition, a voltage to be applied between the cathode 2 and a gate 4 for the purpose of electron emission may be any potential that does not influence the electric field strength due to the anode voltage. Incidentally, FIG. 15A shows an example in which the voltage is set to 0 V in the normally-on state.
On the other hand, during the state shown in FIG. 15B, when a negative potential relative to the cathode 2 is supplied to the gate 4, an electric field strength which the vicinity of the electron emitter 5 receives from the anode 6 becomes small. Accordingly, the electric field strength becomes less than the threshold electric field required for electron emission, whereby electron emission stops. The voltage between the cathode 2 and the gate 4 at this time is called a cut-off voltage.
The equipotential surfaces obtained when the cut-off voltage is applied between the cathode 2 and the gate 4, as shown in FIG. 15B, are 0 V at the cathode 2 and the electron emitter 5, and the gate 4 is at a negative potential. Accordingly, the space between the equipotential surfaces near the electron emitter 5 becomes wide, so that the electric field strength becomes small.
Incidentally, the cut-off voltage applied between the cathode 2 and the gate 4 at this time is suitably determined by the electric field strength required to stop electron emission, and the design of the dimensions of the electron emitter 5, a cathode-gate distance, the dimensions of the gate and the like. The electric field strength required to stop electron emission is determined by the threshold electric field of the electron emitter 5 and the anode voltage relative to the normally-on state.
As described above, in the normally-on type of electron emission element, electron emission is performed by only the application of a voltage between the cathode and the anode. In addition, electron emission is controlled by applying the cut-off voltage between the cathode and the gate and cutting off the electron emission. Accordingly, the voltage between the cathode and the gate need not be made higher than the threshold required for electron emission, whereby low-voltage stable driving control can be realized.
Proposals are made with respect to the art of applying such a normally-on type of electron emission element to an XY matrix type of flat-panel display device. In the case of this type of flat-panel display device, a voltage capable of giving an electric field strength not lower than the threshold of electron emission is applied between the cathodes and the anodes, and while the cut-off voltage is not being applied between the cathodes and the gates, full-screen white display is performed at the maximum luminance on the entire display screen.
Accordingly, in the case where this flat-panel display device is used in a television set or a computer monitor, if full-screen white display is performed even for a short time, a user often mistakes such white display for a failure of the display device or feels uncomfortable.
Particularly when display is completed, for example, when the power source of the display device body is turned off, or when the display device transfers from a display mode to a power-saving non-display mode, or when the power source is shut off by a power failure, even if the anode potential is immediately cut off, the anode potential does not sharply decrease, because electric charge is accumulated on the anode. In addition, since the application of the cut-off voltage is also stopped at this time, the display device continues electron emission until the anode potential decreases below the threshold. Accordingly, during the end period of display, the display device performs full-screen white display at the maximum luminance until the anode potential decreases below the threshold.